Solution-based deposition process for metal chalcogenides

ABSTRACT

A solution of a hydrazine-based precursor of a metal chalcogenide is prepared by adding an elemental metal and an elemental chalcogen to a hydrazine compound. The precursor solution can be used to form a film. The precursor solutions can be used in preparing field-effect transistors, photovoltaic devices and phase-change memory devices.

TECHNICAL FIELD

The present disclosure relates to preparing solutions of metalchalcogenide precursors and especially to preparing solutions in ahydrazine compound. The present disclosure is specifically concernedwith preparing the solutions of the metal chalcogenide precursorswithout the need to first prepare a metal chalcogenide.

The present disclosure also relates to methods of depositing a film ofthe metal chalcogenide. The present disclosure also relates tofield-effect transistors, photovoltaic devices and phase-change memorydevices containing the metal chalcogenide films, as well as to methodsof preparing the field-effect transistors, photovoltaic devices andphase-change memory devices.

BACKGROUND ART

Recently, certain processes have been described for depositing thinfilms of metal chalcogenides. For example, see U.S. Pat. No. 6,875,661to Mitzi, entitled “Solution Deposition of Chalcogenide Films; US PatentPublication 2005-0009225 to Mitzi et al., filed Mar. 16, 2004 entitled“Hydrazine-Free Solution Deposition of Chalcogenide Films,” and USPatent Application Publication 2005-0158909 to Mitzi et al and entitled“Solution Depositon of Chalcogenide Films Containing Transition Metals,”the entire disclosures of which are incorporated herein by reference.

The thin films of the metal chalcogenides are deposited using solutionsprepared by dissolving a metal chalcogenide material in a hydrazine orhydrazine-like solvent. The metal chalcogenide may be of the form MX,MX₂, M₂X₃ or M₂X where M=metal (e.g., Sn, Ge, Pb, In, Sb, Hg, Ga, Tl, K)or a combination thereof and X=chalcogen (e.g., S, Se, Te) or acombination thereof. Since the metals often have the potential formultiple oxidation states, the metal chalcogenide may often benon-stoichiometric and may therefore more generally be represented asM_(y)X_(z) (where 0<y, z and may be an integer or non-integer).

The above-described processes for shorthand purposes can be referred toas hydrazine-precursor techniques or processes. The hydrazine-precursortechnique has the advantage of being a high-throughput process, whichdoes not require high temperatures or high vacuum conditions for thethin-film deposition process. The hydrazine precursor process therebyhas the potential for being low-cost and suitable for deposition on awide range of substrates, including those that are flexible. As metalchalcogenides can exhibit a wide range of electronic character, it maybe used to prepare high-quality semiconducting, insulating or metallicfilms. The process has been used to deposit, for example, both n- andp-type semiconducting films for use as channel layers in thin-filmtransistors (TFTs), exhibiting field-effect mobilities>10cm²N-s—approximately an order of magnitude better than previous resultsfor spin-coatable semiconductors [“High Mobility UltrathinSemiconducting Films Prepared by Spin Coating, Nature, vol. 428, 299(2004)].

Besides TFTs, other electronic devices that rely on metal chalcogenidefilms can also be prepared using the described technique. Solar cells,for example, may consist of thin n-type chalcogenide semiconductorlayers (˜0.25 μm) deposited on a p-type substrate, with electricalcontacts attached to each layer to collect the photocurrent.Light-emitting diodes (LEDs) are typically comprised of a p-n bilayer,which under proper forward bias conditions emits light.

Rewriteable phase-change memory generally employs a film of achalcogenide-based phase-change material, which must be switchablebetween two physical states (e.g., amorphous-crystalline, crystallinephase 1-crystalline phase II). The state of the phase change materialmust also be detectable using some physical measurement (e.g., opticalabsorption, optical reflectivity, electrical resistivity, index ofrefraction). As an example, commercially-available rewritable opticalmemory generally relies on a film of a metal chalcogenide material suchas Ge₂Sb₂Te₅ or KSb₅S₈ [“KSb₅S₈: A Wide Bandgap Phase-Change Materialfor Ultra High Density Rewritable Information Storage,” Adv. Mater.,vol. 15, 1428, 2003]. Initially the film is amorphous, but may beconverted to a crystalline form using a laser beam of sufficientintensity to heat the material above the crystallization temperature.Subsequent exposure to a more intense and short laser pulse melts thecrystallized chalcogenide phase-change material, resulting in aconversion to an amorphous state upon quenching. A recorded bit is anamorphized mark on a crystalline background. The reversibility of thecrystallization-amorphization process allows for the fabrication ofrewritable memory [A. V. Kolobov, “Understanding the phase changemechansim of rewritable optical media, Nature Mater., vol. 3, 703,2004].

Generally the chalcogenide materials in the above-described applicationsare deposited using vacuum-based techniques such as sputtering orthermal evaporation. A solution-based process is desirable because ofthe reduced complexity of the process (reducing cost and improvingthroughput) and the ability to deposit on a wider range of substratetypes (including those that have very large area or are flexible) andsurface morphologies.

One disadvantage of the above-described hydrazine-precursor process isthat it requires the isolation of the metal chalcogenide before thedeposition-process can be initiated. Metal chalcogenides are oftenformed using high-temperature (energy-intensive) and/or multi-step(time-consuming) reactions. Given, the prevalence of multiple possiblecompositions for a given M and X, the formation of single phase metalchalcogenide starting materials can also be problematic. As an example,tin sulfide can exist as SnS or as SnS₂, or perhaps more appropriatelyas SnS_(2-x) to accomodate the potential for non-stoichiometry[“Preparation and Characterization of SnS₂,” J. Solid State Chem., vol.76, 186]. The reaction of a 1:2 (molar) ratio of Sn and S at hightemperature often yields SnS₂, in addition to impurities of SnS and S.The use of these SnS₂ materials for thin-film deposition may thereforelead to non-reproducibility since the exact composition of the startingmetal chalcogenide may vary from run to run.

SUMMARY

The present disclosure relates to a method for preparing a solution of ahydrazine-based precursor of a metal chalcogenide which comprises addingan elemental metal and an elemental chalcogen to a hydrazine compound.

Another aspect of the present disclosure relates to a method ofdepositing a film of a metal chalcogenide which comprises:

-   -   preparing a solution of a hydrazine-based precursor of a metal        chalcogenide which comprises adding an elemental metal and an        elemental chalcogen to a hydrazine compound to provide a        solution of said precursor of the metal chalcogenide;    -   applying a solution of said precursor onto a substrate to        produce a film of said precursor; and annealing the film of the        precursor to produce the metal chalcogenide film on the        substrate.

The present disclosure also relates to a film prepared by the abovedisclosed method.

Another aspect of this disclosure is concerned with a method ofpreparing an improved field-effect transistor of the type having asource region and a drain region, a channel layer extending between thesource region and the drain region, the channel layer including asemiconducting material, a gate region, disposed in spaced adjacency tothe channel layer, an electrically insulating layer between the gateregion the source region, drain region and channel layer. The methodcomprises: preparing a channel layer comprising a film of metalchalcogenide semiconducting material which comprises preparing asolution of a hydrazine-based precursor of a metal chalcogenide byadding an elemental metal and an elemental chalcogen to a hydrazinecompound to provide a solution of said precursor of the metalchalcogenide; applying a solution of said precursor onto a substrate toproduce a film of said precursor; and annealing the film of theprecursor to produce the metal chalcogenide film on said substrate.

The present disclosure also relates to field-effect transistors obtainedby the above method.

A still further aspect of the present disclosure relates to aphotovoltaic device containing a layer or film of a metal chalcogenideobtained by the above-disclosed method.

Another aspect of the present disclosure is concerned with aphase-change memory device containing as a recording layer, a metalchalcogenide obtained by the above-disclosed method.

A still further aspect of the present disclosure relates to preparing abulk metal chalcogenide by heating the metal chalcogenide precursorobtained by the above-disclosed method.

Still other objects and advantages of the present disclosure will becomereadily apparent by those skilled in the art from the following detaileddescription, wherein it is shown and described only in the preferredembodiments, simply by way of illustration of the best mode. As will berealized, the disclosure is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, without departing from the disclosure. Accordingly, thedescription is to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a thermogravimetric analysis (TGA) scan for thehydrazine-based KSb₅S₈ precursor.

FIG. 2 shows powder X-ray diffraction patterns (acquired at a wavelengthof λ=1.54 Å) for the KSb₅S₈ precursor after TGA runs terminated ateither 230° C. or 325° C., demonstrating the formation of crystallineKSb₅S₈.

FIG. 3 illustrates variable-temperature (warming curve) X-raydiffraction (λ=1.797 Å) of the KSb₅S₈ films deposited using the methoddescribed in this disclosure.

FIG. 4(a), 4(b) and 4(c) illustrates three cycles ofvariable-temperature x-ray diffraction (λ=1.797 Å) of a KSb₅S₈ filmafter melting and quenching (warming curves; same film as in FIG. 3).

FIG. 5 is a thermogravimetric analysis (TGA) scan for thehydrazine-based tin sulfide precursor.

FIG. 6 is a powder X-ray diffraction pattern for the SnS₂ precursorafter a TGA run terminated at 350° C. (from FIG. 5), demonstrating theformation of crystalline SnS₂.

FIG. 7 is a schematic of the thin-film transistor (TFT) with aspin-coated tin sulfide channel.

FIG. 8 illustrates Drain current, I_(D), versus drain voltage, V_(D), asa function of gate voltage, V_(G), for a spin-coated tin sulfide channellayer deposited according to this disclosure.

FIG. 9 is a plot of I_(D) and I_(D) ^(1/2) versus V_(G) at a constantV_(D)=16V, used to calculate current modulation, I_(on)/I_(off), andsaturation regime mobility, μ_(sat), for a tin sulfide channel. Thedashed line indicates the slope used to calculate mobility in thesaturation regime.

FIG. 10 is a thermogravimetric analysis (TGA) scan for thehydrazine-based Ga₂Se₃ precursor.

FIG. 11 is a powder X-ray diffraction pattern for the Ga₂Se₃ precursorafter a TGA run terminated at 375° C., demonstrating the formation ofcrystalline Ga₂Se₃.

BEST AND VARIOUS MODES

The present disclosure relates to preparing a solution of a precursor ofa metal chalcogenide. The method comprises adding an elemental metal andan elemental chalcogen to a hydrazine compound. Examples of suitablemetals are both the transition and non-transition, metals and includetin, germanium, lead, indium, antimony, mercury, gallium, thallium,potassium, copper, iron cobalt, nickel, manganese, tungsten, molybdenum,zirconium, hafnium, titanium, and niobium or a combination thereof. Thechalcogen is typically S, Se, Te or a combination thereof.

The elemental metal and elemental chalcogen are typically added inapproximately stoichiometric amounts or with an excess of chalcogen,typically no more than about 150% more than needed for a stoichiometricmixture.

The present disclosure results in replacing the metal chalcogenide usedin prior techniques with the corresponding elemental metal. Theelemental metal combined with sufficient chalcogen results indissolution of the metal in a hydrazine compound. The surprising aspectof this is that many metals may be directly dissolved in ahydrazine-type solvent at room temperature when sufficient chalcogen isadded to the solvent. There is no need to first isolate the metalchalcogenide for use in the preparation of the chalcogenide precursorsolution.

The present disclosure makes possible a high throughput/low-temperaturesolution-based deposition of high quality metal chalcogenide films for avariety of electronic applications (e.g., phase-change memory, solarcell, LED and thin film transistor).

Moreover, the present disclosure differs significantly from the earlierdisclosed use of hydrazine hydrate as a solvent for the precipitation ofcertain metal sulfides and selenides (e.g., zinc sulfide, copperselenide, silver-doped zinc sulfide, copper-doped zinc cadmium sulfide)[U.S. Pat. No. 6,379,585 to Vecht et al., “Preparation of Sulfides andSelenides”]. In the case of U.S. Pat. No. 6,379,585, the solvent (whichalways involves water, as well as hydrazine) generally enables theprecipitation of a metal chalcogenide, rather than the dissolution ofthe metal chalcogenide for further solution processing into thin filmform. In addition, that process did not involve the use of elementalmetals, but rather required metal salts, which could introduceimpurities into the final product.

The solutions of metal chalcogenide hydrazine-based precursors can beprepared by stirring the elemental metal (e.g., Sn, Ge, Pb, In, Sb, Hg,Ga, Tl, K, Rb and Cs) or combinations of elemental metals in a mixtureof hydrazine and chalcogen (e.g., S, Se, Te) to yield the desired metalchalcogenide solution. It is often desirable to add extra chalcogen (inexcess of a stoichiometric quantity to produce the desired M_(y)X_(z)chalcogenide system) to the solvent to improve the solubility of themetal in the hydrazine-based solvent. Generally the stirring isperformed at room temperature, although gentle heating such as up toabout 95 C may also facilitate dissolution. In the case a highlyreactive metal such as potassium, it might be desirable to carry out thestirring (i.e. the reaction) at reduced temperatures for instance downto about 5 C.

Certain of the metals such as potassium cause a highly exothermicreaction to occur upon contact with hydrazine.

One method to overcome the highly exothermic nature of the reactionbetween potassium and hydrazine is to have the potassium physicallyremoved from the bottom of the reaction vessel (e.g., K is “sticky” atroom temperature and will effectively stick to the side of the glasswalls of the reaction flask). Then, when the hydrazine drops are placedon the bottom of the reaction flask, the vapors can first be allowed toreact, followed by gentle agitation of the vessel, allowing some of thedrop to gradually come into contact with the remaining potassium.Further techniques to accommodate the highly exothermic nature of thereaction are to dilute the hydrazine with an appropriate cosolvent suchas an alkanol amine and/or to cool the reaction flask.

The concentration of the metal chalcogenide precursor in the hydrazinecompound is typically no more than about 10 molar and more typicallyabout 0.01 molar to about 10 molar, even more typically about 0.05 toabout 5 molar, or about 0.05 to about 1 molar.

Typical hydrazine compounds are represented by the formula:R¹R²N—NR³R⁴

Wherein each of R^(1,)R²,R³ and R⁴ is independently hydrogen, aryl suchas phenyl, a linear or branched alkyl having 1-6 carbon atoms such asmethyl, ethyl or a cyclic alkyl of 3-6 carbon atoms.

The most typical solvent is hydrazine. The present disclosure is notlimited to the use of hydrazine, but it can also be used withhydrazine-like solvents, as disclosed above, such as1,1-dimethylhydrazine and methylhydrazine or mixtures of hydrazine-likesolvents with other solvents including, but not limited to, water,methanol, ethanol, acetonitrile and N,N-dimethylformanide. However, withcertain highly-reactive metals, e.g. K and other alkali metals, it ispreferred that the solvent be anhydrous.

The metal chalcogenide can be formed from its precursor, afterevaporating the solvent, by heating typically up to about 100-500° C.and more typically up to about 150-350° C. The precursor solutions canadvantageously be used towards the preparation of bulk metalchalcogenides that find particular applicability in phase-change memoryapplications. For instance using the solution-based technique of thisdisclosure, bulk metal chalcogenides can be prepared at temperatures<350 C after evaporation of the solvent and typically at temperatures ofabout 150 to <250 C. On the other hand, the preparation of, for example,bulk KSb₅S₈ is generally carried out at ˜850 C [“KSb₅S₈: A Wide BandgapPhase-Change Material for Ultra High Density Rewritable InformationStorage,” Adv. Mater., vol. 15, 1428, 2003].

The solution of the metal chalcogenide precursor can be applied to asubstrate. The substrate may be rigid or flexible and the depositionprocess may be by any solution-based technique including, but notlimited to, doctor blading, spin coating, ink-jet printing, stamping,dip-coating.

Typically, the substrate is fabricated from a material having at leastone property selected from the following: thermally stable, i.e., stableup to about at least 300° C.; chemically inert towards the metalchalcogenides; rigid or flexible. Suitable examples include Kapton,silicon, amorphous hydrogenated silicon, silicon carbide (SiC), silicondioxide (SiO₂), quartz, sapphire, glass, metal, diamond-like carbon,hydrogenated diamond-like carbon, gallium nitride, gallium arsenide,germanium, silicon—germanium, indium tin oxide, boron carbide, boronnitride, silicon nitride (Si₃N₄), alumina (Al₂O₃), cerium (IV) oxide(CeO₂), tin oxide (SnO₂), zinc titanate (ZnTiO₂), a plastic material ora combination thereof. More typically, the metal substrate is a metalfoil, such as, aluminum foil, tin foil, stainless steel foil and goldfoil, and the plastic material more typically is polycarbonate, Mylar orKevlar.

The hydrazine-based precursor on the substrate is then subjected to alow-temperature thermal treatment or annealing to decompose theprecursor to the metal chalcogenide.

The thermal treatment is typically between 100-500° C. (more typically,between 150-350 C) and for an amount of time just sufficient todecompose the precursor and effect sufficient grain growth. In somecases (for certain applications that might require an amorphous film),the thermal treatment is carried out at an appropriate temperature thatis high enough for thermal decomposition of the precursor, but lowenough to enable the film to remain in an amorphous state. Typically,the thermal treatment is for an amount of time between 1 sec-1 hr. Moretypically, the thermal treatment is for 5-30 min. The thermal treatmentmay be applied using a hot plate, oven (tube- or box-type), laser-basedrapid annealing or microwave-based heating. The thermal treatment yieldsan amorphous or crystalline film of the desired metal chalcogenide, withthe loss of hydrazine and hydrazinium chalcogenide (and/or thedecomposition products of these compounds).

The present disclosure has the advantage relative to U.S. Pat. No.6,875,661 and Patent Publications 2005-0009225 and 2005-0158909,mentioned above that it is not necessary to purchase or synthesize themetal chalcogenide material, which often requires high temperatureand/or multiple step synthetic processes for its preparation. Rather thecorresponding metal is simply employed High purity samples of mostmetals are readily available from chemical suppliers (the correspondingmetal chalcogenides are sometimes difficult to purchase commercially).Generally, much higher purity metals can be purchased, when comparedwith the corresponding purchased metal chalcogenide. Betterreproducibility of the properties of the resulting films is thereforeexpected.

The present disclosure may be used for the preparation of materials orcomponents for a variety of electrical devices, including phase-changememory devices (e.g., optical rewritable memory or PRAM), transistors,solar cells or LEDs.

The films prepared by the present disclosure can be removed from thesubstrate to produce an isolated film thereof.

The present disclosure provides a thin-film field-effect transistor(FET) having a film of a metal chalcogenide semiconducting material asthe active semiconducting layer. The present disclosure provides amethod of preparing an improved field-effect transistor of the typehaving a source region and a drain region, a channel layer extendingbetween the source region and the drain region, the channel layerincluding a semiconducting material, a gate region disposed in spacedadjacency to the channel layer, an electrically insulating layer betweenthe gate region and the source region, drain region and channel layer,wherein the method includes: preparing a channel layer including a filmof a solution-processed metal chalcogenide semiconducting material ofthe present disclosure.

In one embodiment, the source region, channel layer and drain region aretypically disposed upon a surface of a substrate, the electricallyinsulating layer is disposed over the channel layer and extending fromthe source region to the drain region, and the gate region is disposedover the electrically insulating layer, for example, as shown in FIG. 4of the U.S. Pat. No. 6,180,956, the disclosure of which are incorporatedherein by reference.

In another embodiment, the gate region is disposed as a gate layer upona surface of a substrate, the electrically insulating layer is disposedupon the gate layer, and the source region, channel layer, and drainregion are disposed upon the electrically insulating layer, for example,as shown in FIG. 3 of the previously incorporated U.S. Pat. No.6,180,956.

The metal chalcogenide semiconducting material may be in the form of athin film, in which the metal chalcogenide semiconducting material is apolycrystalline material having a grain size equal to or greater thanthe dimensions between contacts in the semiconductor device.Accordingly, the present disclosure can provide an improved field-effecttransistor prepared by the aforementioned method.

Photovoltaic cells might be constructed, incorporating the variousmethods of this disclosure, by layering the metal chalcogenide withother materials to form a two terminal, sandwich-structure device. Forexample, one could form a layer of CuInS_(x)Se_(2-x), deposited asdisclosed herein on top of a metal contact, such as Mo which issupported on a rigid or flexible substrate (e.g. glass, metal, plastic).The CuInS_(x)Se_(2-x) layer could then be covered with a buffer layer,which can be a metal chalcogenide such as CdS or ZnSe or an oxide suchas TiO₂. This buffer layer could be deposited in the same fashion as theCuInS_(x)Se_(2-x) layer using any of the methods of the presentdisclosure or it could be deposited more conventionally (e.g. bychemical bath or vapor deposition techniques). The buffer layer wouldthen be covered with a transparent top contact such as doped TiO₂,indium tin oxide, or fluorine-doped tin oxide, completing thephotovoltaic cell.

Alternatively, the cell could be constructed in the reverse order, usinga transparent substrate (e.g. glass or plastic) supporting a transparentconducting contact (such as doped TiO₂, indium tin oxide, orfluorine-doped tin oxide). The buffer layer would then be deposited onthis substrate and covered with the metal chalcogenide layer (such asCuInS_(x)Se_(2-x) or CdTe), and finally with a back contact (such as Moor Au). In either case, the buffer layer and/or the metal chalcogenide(“absorber”) layer could be deposited by the methods described in thisdisclosure.

Metal chalcogenides obtained by the above-disclosed method can also beused as a recording layer in phase change memory devices such as thosedisclosed and referenced in WO 2004/067624, disclosure of which isincorporated herein by reference. The phase change memory devicestypically include the chalcogenide-based phase-change memory materialsupported on a substrate and protected by other insulating or conductinglayers (depending upon the application).

In the addition, the process of the present disclosure can be combinedwith the modes disclosed in U.S. Ser. No. 11/330,291 filed Jan. 12, 2006and entitled “Method for Fabricating an Inorganic NanoComposite, entiredisclosure of which is incorporated herein by reference, to enable thedeposition of inorganic nanocomposites comprising a metal chalcogenidematrix molecularly interspersed with nanoentities (nanorods,nanoparticles, nanowires, nanotetrapods, etc.). In this case, thenanoentities would be added to the metal chalcogenide precursor solutionbefore depositing the film.

The following non-limiting examples are presented to further illustratethe present disclosure.

EXAMPLE 1 KSb₅S₈

Under rigorously inert atmosphere conditions, 0.5 mmol of elemental K(19.6 mg; Alfa Aesar, 99.95%, ampouled under Ar) are combined with 2.5mmol Sb (304.4 mg; Alfa Aesar; 99.999%,-200 mesh), 8.0 mmol S (256.5 mg;Aldrich, 99.998%) and 1.0 mL anhydrous distilled hydrazine. Thehydrazine is added very carefully (drop-by-drop and very slowly) toaccommodate the highly exothermic reaction. The mixture is stirred for 5days at room temperature in a nitrogen-filled dry box, forming anessentially clear relatively viscous yellow solution (a tiny amount ofblack precipitate or undissolved material is still present but can beeasily removed using a filter). For bulk analysis of the precursor, thesolution is evaporated under flowing nitrogen gas and under vacuum,yielding a very viscous darkly-colored gum-like product. Thermaldecomposition of the product (FIG. 1) yields the phase-change materialKSb₅S₈ (FIG. 2). FIG. 1 is a thermogravimetric analysis (TGA) scan forthe hydrazine-based KSb₅S₈ precursor, performed using a 1° C./minheating rate and a flowing nitrogen atmosphere. FIG. 2 shows powderX-ray diffraction patterns (λ=1.54 Å) for the KSb₅S₈ precursor after TGAruns terminated at either 230° C. or 325° C., demonstrating theformation of crystalline KSb₅S₈. The bottom curve in FIG. 2 is acalculated diffraction pattern, based on a published single crystalstructure for KSb₅S₈ [The Crystal Structures of Synthetic KSb₅S₈ and(Tl_(0.598)K_(0.402))Sb₅S₈ and Their Relation to Parapierrotite (TlSb₅S₈), Z. Kristallogr., vol. 214, 57, 1999]. The thermal decompositioncorresponds to the loss of approximately 36.4% of the initial precursorweight. Note also that the thermal decomposition is complete byapproximately 220° C., rendering this a fairly low-temperature process.

One method to overcome the highly exothermic nature of the reactionbetween potassium and hydrazine is to have the potassium physicallyremoved from the bottom of the reaction vessel (e.g., K is “sticky” atroom temperature and will effectively stick to the side of the glasswalls of the reaction flask). Then, when the hydrazine drops are placedon the bottom of the reaction flask, the vapors can first be allowed toreact, followed by gentle agitation of the vessel, allowing some of thedrop to gradually come into contact with the remaining potassium.Further techniques to accommodate the highly exothermic nature of thereaction are to dilute the hydrazine with an appropriate cosolventand/or to cool the reaction flask.

KSb₅S₈ is both a potentially important phase-change material, as well asan example of a ternary compound being synthesized using the process.

EXAMPLE 2 Thin Film Deposition of KSb₅S₈

For thin-film deposition, the above-described solution proved tooviscous for effective thin-film deposition by spin coating. The solutionis therefore diluted by adding an additional 3 mL of distilledhydrazine. Films can then be spin coated from the hydrazine-basedsolution onto Si substrates (2 cm×2 cm), coated with approximately 100nm of thermal oxide (SiO_(x)). The substrates are cleaned using aPiranha process (4:1 H₂SO₄: 30% H₂O₂ by volume) to provide a cleanhydrophilic surface so that the KSb₅S₈ solution adequately wets thesurface during the spinning process. The films are spin coated in anitrogen-filled drybox by depositing two drops of the chalcogenide-basedsolution on the substrate and spin coating at between 2500-5000 rpm. Theexact spin speed influences the thickness of the resulting films. Afterspin coating, the resulting films are dried at 100° C. for approximately5 min, then gradually heated to 225° C. over a period of 10 min andmaintained at this temperature for 15 min. This heat treatmentdecomposes the precursor, resulting in the desired crystalline KSb₅S₈films.

The KSb₅S₈ films, deposited using the hydrazine-based solution asdescribed above, exhibit the expected phase-change properties. FIG. 3shows a segment (2θ range) of the powder X-ray diffraction pattern (FIG.2), as a function of temperature. FIG. 3 is a variable-temperature(warming curve) X-ray diffraction pattern (λ=1.797 Å) of the KSb₅S₈film, deposited using the method described in this disclosure (using2500 rpm spin speed). The film was rapidly cooled to room temperatureafter melting. The diffraction pattern effectively disappears above˜448° C., the expected melting temperature of KSb₅S₈ [“KSb₅S₈: A WideBandgap Phase-Change Material for Ultra High Density RewritableInformation Storage,” Adv. Mater., vol. 15, 1428 (2003)]. Upon cooling,the film is effectively quenched into an amorphous state (i.e., nodiffraction peaks at low temperature in FIG. 4 a). Warming the resultingamorphous films leads to an amorphous-crystallization transition at ˜287C, consistent with the literature value (FIG. 4). Above the meltingpoint, the film again enters the amorphous state. Three thermal cycles,FIG. 4(a), 4(b) and 4(c) are shown, demonstrating the reversibility ofthe amorphous-to-crystalline transition. FIG. 4 is avariable-temperature X-ray diffraction pattern (λ=1.797 Å) of a KSb₅S₈film after melting and quenching (warming curves; same film as in FIG.3). The film was subsequently quenched to room temperature after eachwarming segment. This is proof that the films indeed consist of aphase-change material, potentially suitable for use in rewritableoptical memory and other phase-change memory applications.

EXAMPLE 3 SnS₂

2 mmol elemental Sn (237.4 mg; Aldrich, tin shot, 99.999%) are combinedwith 8 mmol elemental S (256.5 mg; Aldrich, 99.998%) and 4 mL anhydrousdistilled hydrazine. The mixture is stirred at room temperature in anitrogen-filled glove box (water and oxygen levels below 1 ppm) for aperiod of approximately 4 days, yielding a clear very pale yellowsolution. The solution may be used for solution-processing of tinsulfide films and devices as described below. For bulk characterization,the solution was evaporated under flowing nitrogen gas for approximately18 hr and further dried under vacuum, yielding approximately 591 mg of awhite or pale yellow powder. Upon thermal decomposition of the precursorpowder (˜40.5% weight loss), SnS₂ (or SnS_(2-x)) is recovered (FIGS. 5and 6). FIG. 5 is a thermogravimetric analysis (TGA) scan for thehydrazine-based tin sulfide precursor, performed using a 2 C/min heatingrate and a flowing nitrogen atmosphere. FIG. 6 is a powder X-raydiffraction pattern for the SnS₂ precursor after a TGA run terminated at350 C (from FIG. 5), demonstrating the formation of crystalline SnS₂.The reflection indices are taken from the known powder pattern forBerndtite. Relatively broad peaks in the X-ray diffraction pattern (FIG.6) indicate a small grain size for the polycrystalline product.

EXAMPLE 4 Thin Film Deposition of SnS₂

Thin tin sulfide films are also deposited from an analogous solution tothat described above. 0.12 mmol of Sn (14.24 mg; Aldrich, tin shot,99.999%) are combined with 0.50 mmol S (16.03 mg; Aldrich, 99.998%) and1.8 mL hydrazine (distilled). The mixture is stirred for approximately 2days, yielding an essentially colorless solution. The films arespin-coated on Si substrates (2 cm×2 cm), each coated with approximately40 nm of thermal oxide (SiO_(x)), which have been cleaned using aPiranha process (4:1 H₂SO₄: 30% H₂O₂ by volume). The important aspect ofthe cleaning process is to provide a clean hydrophilic surface so thatthe tin sulfide solution will adequately wet the surface during thespinning process. The films are spin coated in a nitrogen-filled dry boxby depositing two drops of the chalcogenide solution on the substrateand spin coating at between 1000-4000 rpm. The exact spin speedinfluences the thickness of the resulting film. After spin coating theresulting films are dried at 120° C. for approximately 5 min and thenheat treated at between 250-350° C. to affect the thermal decompositionof the precursor to tin sulfide. The final annealing temperature canalso be used to tailor the grain size of the film and influence theresulting film electrical characteristics.

EXAMPLE 5 Field Effect Transistor

A characteristic tin sulfide film, prepared as described above with a3000 rpm spin speed and a 325 C final heat treatment temperature wasused as the channel layer in a thin-film transistor (TFT). Thetransistor comprises (see FIG. 7) a heavily n-doped silicon wafer as thesubstrate/gate, a 40-nm-thick thermally grown SiO₂ gate dielectric, thespin-coated tin sulfide channel layer and patternedelectron-beam-evaporated Au source and drain electrodes.

A representative plot of drain current, I_(D), versus drain voltage,V_(D), is shown in FIG. 8 as a function of the applied gate voltage,V_(G), for a TFT with a tin sulfide channel formed from the solutiondescribed above (the channel length, L=25 μm, and channel width, W=1000μm).

The device operates as an n-channel transistor, operating inaccumulation mode upon application of a positive gate bias. Applicationof a negative gate bias depletes the channel of electrons and shuts thedevice off. At low V_(D), the TFT shows typical transistor behavior asI_(D) increases approximately linearly with V_(D). Current saturation,with a small ohmic component, is observed at high V_(D) as theaccumulation layer is pinched off near the drain electrode. Currentmodulation (I_(ON)/I_(OFF)) and saturation regime field-effect mobility(μ_(sat)) are calculated from the plot of I_(D) and I_(D) ^(1/2) versusV_(G) (FIG. 9), yielding I_(ON)/I_(OFF)=2×10⁴ and μ_(sat)=0.14 cm²/V-s,respectively, for a −4 to 16 V gate sweep and V_(D)=16 V. The dashedline in FIG. 9 indicates the slope used to calculate mobility in thesaturation regime. Note that use of a thinner or higher dielectricconstant gate insulator (relative to the 400 Å SiO₂ layer currentlyused) is expected to enable reduction in the device operating voltage.The linear regime mobility derived from the plots in FIG. 8,μ_(lin)=0.12 cm²/V-s, is only slightly lower than the saturation regimevalue.

EXAMPLE 6 Ga₂Se₃

1.5 mmol of elemental Ga (104.58 mg) are combined with 3 mmol elementalSe (236.88 mg) and 2 mL anhydrous hydrazine. The mixture is stirred atroom temperature in a nitrogen-filled glovebox (water and oxygen levelsbelow 1 ppm) for a period of two weeks. The initially dark solutioneventually becomes a clear yellow with only a very small piece of theundissolved metal on the bottom of the reaction flask. The resultingyellow solution can be used (as described above for SnS₂ and KSb₅S₈) forthe deposition of Ga₂Se₃ films. Bulk Ga₂Se₃ is formed by evaporating thesolution and heating the resulting precursor to 375° C. (FIGS. 10 and11). FIG. 10 is a thermogravimetric analysis (TGA) scan for thehydrazine-based Ga₂Se₃ precursor, performed using a 1 C/min heating rateand a flowing nitrogen atmosphere. FIG. 11 is a powder X-ray diffractionpattern (λ=1.54 Å) for the Ga₂Se₃ precursor after a TGA run terminatedat 375 C, demonstrating the formation of crystalline Ga₂Se₃. Thereflection indices are taken from the known powder pattern for α-Ga₂Se₃.

The foregoing description illustrates and describes the presentdisclosure. Additionally, the disclosure shows and describes only thepreferred embodiments of the disclosure, but, as mentioned above, it isto be understood that it is capable of changes or modifications withinthe scope of the concept as expressed herein, commensurate with theabove teachings and/or skill or knowledge of the relevant art. Theembodiments described hereinabove are further intended to explain bestmodes known of practicing the invention and to enable others skilled inthe art to utilize the disclosure in such, or other, embodiments andwith the various modifications required by the particular applicationsor uses disclosed herein. Accordingly, the description is not intendedto limit the invention to the form disclosed herein. Also, it isintended that the appended claims be construed to include alternativeembodiments.

All publications, patents and patent applications cited in thisspecification are herein incorporated by reference, and for any and allpurposes, as if each individual publication, patent or patentapplication were specifically and individually indicates to beincorporated by reference. In the case of inconsistencies, the presentdisclosure will prevail.

1. A method of preparing a solution of a hydrazonium precursor of ametal chalcogenide which comprises adding an elemental metal and anelemental chalcogen to a hydrazine compound.
 2. The method of claim 1wherein the concentration of the metal chalcogenide precursor in thehydrazine compound is no more than about 10 molar.
 3. The method ofclaim 1 wherein the concentration of the metal chalcogenide precursor inthe hydrazine compound is about 0.01 molar to about 10 molar.
 4. Themethod of claim 1 concentration of the metal chalcogenide precursor inthe hydrazine or about 0.05 to about 1 molar.
 5. The method of claim 1wherein the hydrazine compound is represented by the formula;R¹R²N—NR³R⁴ wherein each of R^(1,)R²,R³ and R⁴ is independentlyhydrogen, aryl, a linear or branched alkyl having 1-6 carbon atoms or acyclic alkyl of 3-6 carbon atoms.
 6. The method of claim 1 wherein thehydrazine compound comprises hydrazine.
 7. The method of claim 1 whichcomprises reacting vapors of the hydrazine compound with the elementalmetal.
 8. The method of claim 1 wherein said metal chalcogenidecomprises KSb₅S₈.
 9. The method of claim 1 wherein said metalchalcogenide comprises SnS₂.
 10. The method of claim 1 wherein saidmetal chalcogenide comprises Ga₂Se₃.
 11. The method of claim 1 whereinsaid hydrazine compound is substantially anhydrous.
 12. A method ofdepositing a film of a metal chalcogenide which comprises preparing asolution of a hydrazinuim precursor of a metal chalcogenide whichcomprises adding an elemental metal and an elemental chalcogen to ahydrazine compound to provide a solution of said precursor of the metalchalcogenide; applying a solution of said precursor onto a substrate toproduce a film of said precursor; and annealing the film of theprecursor to produce the metal chalcogenide film on the substrate. 13.The method of claim 12 wherein the annealing is carried out attemperatures of about 50 to about 500° C.
 14. The method of claim 12wherein the annealing is carried out at temperatures of about 100 toabout 350° C.
 15. The method of claim 12 wherein the concentration ofthe metal chalcogenide precursor in the hydrazine compound is no morethan about 10 molar.
 16. The method of claim 12 wherein the hydrazinecompound is represented by the formula;R¹R²N—NR³R⁴ wherein each of R^(1,)R²,R³ and R⁴ is independentlyhydrogen, aryl, a linear or branched alkyl having 1-6 carbon atoms or acyclic alkyl of 3-6 carbon atoms.
 17. The method of claim 12 wherein thehydrazine compound comprises hydrazine.
 18. A film prepared by themethod of claim
 12. 19. A method of preparing an improved field-effecttransistor of the type having a source region and a drain region, achannel layer extending between the source region and the drain region,the channel layer including a semiconducting material, a gate region,disposed in spaced adjacency to the channel layer, an electricallyinsulating layer between the gate region and the source region, drainregion and channel layer, wherein the method comprises: preparing achannel layer comprising a film of a metal chalcogenide semiconductingmaterial which comprises preparing a solution of a precursor of a metalchalcogenide by adding an elemental metal and an elemental chalcogen toa hydrazine compound to provide a solution of said precursor of themetal chalcogenide; applying a solution of said precursor onto asubstrate to produce a film of said precursor; and annealing the film ofthe precursor to produce the metal chalcogenide film on said substrate.20. The method of claim 19 wherein the annealing is carried out at atemperature of about 50 to about 500° C.